|CWS Defiant A55 Germanium Transistor Radio Set|
My mother and father purchased a radio set, about the time they were married (around 1960).
I remember it from my childhood as it was always on in the mornings.
The radio is a Defiant brand set made by the Cooperative Wholesale Society. There is quite a lot about this type of radio on the internet, so I won't go into the history here. Mine is an A55 model.
It belongs to an era when radios had germanium transistors. With hind-sight we know now that was quite short period, only about a decade, from the end of the fifties to the mid sixties. Valved radio sets were regarded as old fashioned, and the silicon transistor had not yet become commonplace.
I have had the radio up in the loft for many years, and last time I tried it, the audio was quite distorted.
|Small modern capacitors wired in parallel with the old.|
I thought it would be a good idea to replace the electrolytic capacitors. They are the yellow components in the picture at left. They have a red end and a black end to indicate their polarity.
These components deteriorate with age, because they have a liquid electolyte, The seals can fail and the capacitors dry out. This type of capacitor is often used between sections of the circuit, where they allow the signals to pass through (these are a.c. currents at audio frequencies) whilst preventing direct current from flowing, thus keeping each transistor correctly biased.
Modern components are very much smaller than the 1960 types, so I decided to leave the old capacitors in place on the board, cutting one wire so they were no longer in circuit. Then I soldered replacement parts on the underside of the board, thus keeping the appearance of the PCB the same, but hopefully restoring full circuit function.
After replacing all six capacitors, I switched on the set, which initially worked well - but next time I turned it on it didn't. Sometimes it worked, sometimes the audio was weak and distorted. The Defiant has an unusual power-supply arrangement. It was powered from a PP11 type battery, which is fact two 4.5V batteries in the same box. This provided a split rail supply. The circuit runs from 9 Volts, but the speaker is connected to the mid-point of the batteries. This was done to avoid the need for a large electrolytic capacitor in series with the loudspeaker, and ensured a good bass response on the audio. But it also means that the On/Off switch has to be a double-pole type, in order to switch both positive and negative rails. The on/off switch on my Defiant was intermittent on one, or both sides.
|On/Off switch seen here in the "off" position,|
|On/Off switch seen here in the "on" position,|
And here in the "on" position.
Rather than replace the On/Off volume control with a modern part, I took the switch off the back of the potentiometer. This was quite easy to do and just required bending some metal tabs. The switch had some old dried grease in it. I used some spray switch cleaner to soften and remove a lot of the grease, but hopefully leave the internals clean and lubricated. The switch works really well now.
I am happy to report that the set now sounds fantastic, on both long and medium wavebands. BBC radio 4 is still broadcasting on longwave in the UK. 198kHz.
The other things I have done are cleaning – the case was very dirty. I also glued a bit of scrim back over the hole in the back of the cabinet.
The set has 6 transistors, OC45’s in the RF section and an OC75 as an audio driver, with two OC74’s in the output stage. There were also 6 electrolytic capacitors, ranging from 8uF to 250uF.
I have taken the spun brass disks out of the control knobs and polished out the tarnish – then gave them a coat of varnish to prevent them discolouring again.
the disks were quite easy to remove by poking the end of a cocktail stick through the little hole in the plastic moulding. The glue was hard and dry, and I replaced it with a little dab of hot-melt glue.
|Flaky paint on the dial|
Unfortunately the paint on the rear of the dial is flaking off, which is a great shame and difficult to fix. I don’t think there’s much that can be done there.
Since PP11 batteries are no longer available I am using a 6-pack of AA cells, I connected a wire to the mid point of the battery pack to provide the split-rail supply that I mentioned earlier.
The set has a particularly rich and full sound. Here are a few more photos.